The definition of a forum:

For student media to be designated as a public/student forum, the school must either:
• Have a school board- or administrator-enacted policy stating students make final content decisions of protected speech*, or
• Have a student media-generated policy declaring students make all final content decisions and also indicating/verifying that practice has been in effect at least two years, and there is no district or building policy that directly contradicts that practice. During that time, no adult, including the adviser, other faculty members, administrators or publication boards have dictated or changed content
•In both situations, the advisers may, as part of the coaching process, offer advice and comment, but not make final content decisions

* The policy can still limit unprotected speech such as libel, obscenity and substantially disruptive material, but it must give other content control to the students